Fiber-optic cables are used in a wide variety of applications today to replace traditional copper cables. Such fiber-optic cables, for example, may be utilized to transmit data and control signals between computers and processors. Optical fiber provides reliable data transfer, with exceptional speed and bandwidth. The small size and the light weight of fiber-optic cables make them particularly useful in communication applications, which have significant space and weight restrictions. Fiber-optic cables receive significant use in the aerospace industry for both commercial and military applications. In such usage, the fiber-optic cables must have a very robust construction because even minor failures in a cable may have significant undesirable consequences. Generally, the construction of a fiber-optic cable includes a glass strand, or fiber, that is surrounded by one or more outer layers, or jackets. For example, a fiber-optic device may include a glass strand having a suitable cladding for transmitting the optical signals. A coating is applied over the glass strand. A buffer layer is utilized on the outside of the glass strand for physically supporting and buffering the fragile glass strand. Furthermore, since fiber-optic cables are often subjected to extremes in temperature, pressure, vibration and shock, additional layers, such as strength layers, are utilized on the outside of the buffer. Finally, an insulated jacket layer surrounds the entire fiber-optic cable assembly to provide a protective outer surface. Additionally, each of the buffer layers, strength member and jacket layer serves to provide a robust structure in which the attachment of the cable to a terminus and connector can be made.
One particular parameter that is addressed in manufacturing and utilizing fiber-optic cables is the engagement force, that is necessary to mate the terminal ends of two fiber-optic cables, such as in a connector assembly. This force is also sometimes referred to as the “mating force” of a connector, however, the more standard terminology is “engagement force”. The engagement force associated with a fiber-optic cable is of particular interest, as new applications demand an increase in the density of cables that are terminated in a connector. The engagement forces of all the cables in a connector are cumulative, and thus they increase linearly with an increase in the cable density of the connector.
Generally, one or more fiber-optic cables are terminated in a suitable connector, or termini, that is then plugged into or mated with another, appropriate cable connector. To insure a proper interface at the ends of the mated fiber-optic cables, the connectors include spring-loaded contact elements. The spring-loaded elements of the connector in which the fiber-optic cables terminate, must be depressed, or translated, within the connector housing when the connector is mated with another connector. As such, a certain amount of force is required to translate the multiple spring-loaded connector elements of multiple cables terminated in the connector. Such a force contributes to the “engagement force” of the connectors. As may be appreciated, the greater the density of cables at a connector, the greater the engagement force for that connector.
Because of the construction of the connectors and the process of terminating fiber-optic cables therein, portions of the fiber-optic cable, namely the glass fiber and buffer, must telescope, or longitudinally slide, inside one or more of the other cable layers. Specifically, the glass fiber and buffer are terminated at the spring-loaded contact element, while the outer layers of the fiber-optic cable are held stationary with respect to the connector. When the connector is mated with another connector, the spring-loaded contact element moves in the connector body, or housing. As such, when the spring-loaded contact element and the fiber therein, move within the connector housing, the glass fiber and buffer layers generally will move slightly, or telescope, with respect to the strength layers and jacket layers, and any other layers that are terminated at the end of the connector housing. Consequently, there is an additional force required during connector mating that is also necessary to move the glass fiber and buffer, with respect to the other layers of the fiber-optic cable. This force, often referred to as a “buffer insertion force” or “buffer push-in force”, adds to the force that is necessary to move the spring-loaded contact element within the connector housing. For consistency, the terminology “buffer insertion force” will be used throughout the remainder of this document.
While the buffer insertion force for a single fiber-optic cable, or even several cables, may not be a particular issue in connecting the terminal ends of the cables, a desire for greater connector density, particularly in the aerospace industries, has generated a need to reduce the high engagement force that may result from such high density connectors. That is, the cumulative buffer insertion force that increases in a multiplicative fashion as the number and density of fibers in a particular connector increases, may create an engagement force so large that it is difficult for an installer, without additional machines or tools, to connect two opposing connectors. As such, it is desirable to reduce the high engagement forces that result from high-density fiber-optic connectors.
Fiber-optic cables are available that provide desirable performance and durability characteristics, particularly for the aerospace industries. For example, the assignee of the present invention, Tensolite Company, of Saint Augustine, Fla., provides a fiber-optic cable manufactured to Boeing Commercial Aircraft Company's specification, BMS 13-71, which meets the vigorous standards of the aerospace industry. The Tensolite manufactured BMS 13-71 cable uses a glass fiber that includes a fiber-optic core, cladding, and a conventional coating material. It utilizes multiple buffer layers, which include a first buffer layer that is an extruded expanded PTFE (ePTFE). A second buffer layer is formed of two opposing helical wraps of adhesive coated 0.001″ polyimide tapes that are wrapped, and then fused together with heat. On the outside of the buffer layer, an overlap skived 0.001″ thick PTFE tape is loosely wound in a helical wrap. Then, a strength member, such as a braided layer, made of a woven aramid fiber and glass fiber, is positioned over the buffer layers, and PTFE tape. An outer jacket layer made of an extruded fluoropolymer, such as FEP, provides the outer layer of a fiber-optic cable.
Another issue that must be addressed with fiber-optic cables is the kink resistance of the cable. Fiber-optic cables, similar to other cables, are bent and curved and otherwise manipulated when installed. As a result, severe bending or manipulating the cable around a small radius may result in a kink in the cable, thus reducing the light transmission to the point of rendering the cable inoperable. As such, the kink resistance of a cable is an important parameter in determining whether the fiber-optic cable is suitable for a particular application.
It is therefore desirable to improve generally upon existing fiber-optic cable technology and to provide a fiber-optic cable with a significantly-reduced buffer insertion force that makes the cable useful for high-density fiber-optic connectors and applications. It is also desirable to reduce the engagement force necessary for mating high density connectors, while maintaining and improving the overall performance and durability of the fiber-optic cables. It is further desirable to increase the kink resistance of a cable. The present invention addresses these issues and other issues, as set forth in more detail below.
These features and other features of the invention will be come more readily apparent from the Detailed Description and drawings of the application.